Modify Strings2.urlDecode to input a string.

This is consistent with urlEncode.  Also consistently use urlDecoder in
DecodingMultimap for values.
Handle quoted ETags for OpenStack objects to fix JCLOUDS-247

This avoids a 'java.io.IOException: Unrecognized character: "' when a quoted ETag is encountered (which happens when getting a multipart blob from Rackspace Cloud).
Only decode strings that are actually encoded.

Update HttpRequestTests to account for change in urlDecode.

References JCLOUDS-217
References JCLOUDS-200